Versus: Argue why Foo is better than Bar
It was just released near the beginning of the month and is building momentum but there are already very good comparisons with many arguments on both sides:
MySQL vs PostgreSQL
Static typing vs Duck typing
Squeezebox vs AirTunes
Yelp vs Zagat
The other cool thing about it is that anyone can add new arguments, people can vote for arguments on both sides, there are comments associated with each argument, and anyone can introduce a new competition. With luck Versus will become the place to settle arguments of all kinds!